1. Field of the Invention
An object of the present invention is a method for the acquisition, in nuclear medicine, of a scatter image (i.e. an image obtained by scatter as defined here below) of the body of a patient being examined. The scatter image thus obtained may be used as such to reveal the anatomy of the examined body. Preferably, it will be used to correct the results of an emission image (i.e. an image obtained by emission as defined here below) revealed by the same machine. The value of the invention is that, in the latter case, it is possible to acquire both images simultaneously. This results in a gain in time and above all in a far better geometrical comparison of the two images acquired to enable the correction of the results of one of them as a function of those of the other.
2. Description of the Prior Art
Nuclear medicine is a branch of diagnostic medicine in which it is sought to obtain functional information about the bodies of the patients examined. To this end, radioactive markers are injected in these patients. The marker used is generally technetium. This radioactive marker, depending on the biological agent that carries it, will get fixed into one organ or another of the patient. At the place where it is fixed, it sends out radioactive radiation, namely gamma rays. The greater the quantity of markers fixed in the organ, the greater is the number of these rays. This is a phenomenon of emission.
To measure this radioactive phenomenon, a gamma camera or scintillation camera is used. A nuclear medicine machine therefore essentially has a mount to support a gamma camera of this type. This camera has a detector and means to compute the detected image. The detector has a scintillator crystal. Generally, the scintillator crystal is plane. The scintillator absorbs the gamma radioactive radiation. By photo-electrical effect, it emits a luminous scintillation in response. This luminous scintillation is detected downline by an array of photomultiplier tubes of the detector. In addition, a gamma camera has a control and processing panel. The tubes are thus associated with computation means. These means make it possible to determine the coordinates of a locus of interaction of the gamma rays in the scintillator. This locus reveals the projected image of the body.
In view of the fact that the radioactive emission in the body is omnidirectional, this localizing can be carried out efficiently only by the interposing of a collimator between the body and the scintillator. This collimator lets through only the radioactive rays that get propagated in a chosen direction.
With such examinations by emission, it is possible to produce projection images. If the gamma camera is rotated about the patient's body while the radioactive phenomenon occurs (for an examination period of about half an hour), it is possible to acquire a certain number of projections, for example parallel-type projections. With these parallel-type projections it is possible, by means of tomography type methods, to reconstruct sectional images of the body. The projections are of a parallel type because the collimator lets through the rays in only one direction perpendicular to its plane.
However, the mode of acquisition thus briefly recalled has a drawback: the gamma rays emitted by the internal structures of the body must cross other regions of this body before exciting the scintillator and, in these other regions of the body, they undergo a corresponding attenuation. This attenuation disturbs the acquisition of the acquired images and the exactness and precision of the reconstructed images. Many attempts have been made to take account of this attenuation, sometimes without really measuring it. However, the method thus proposed has given few results and, to date, the real measurement of attenuation, notably by transmission tomography, is the only approach that can be envisaged. The knowledge obtained, by such a method, of the attenuation coefficient related to the radiological density at each position of the body makes it possible to correct the results of the images obtained by emission. The correction takes account of the varying size of the mass of the tissues interposed in the path of a radioactive emission coming from a disintegration of the marker.
French patent application No. 89 10225 filed on 28 Jul. 1989 and published under No. 2 650 397, recommends the performance of such a transmission-type measurement of the internal structures of the body with a device comprising an external point source of gamma radiation facing the detector of the gamma camera. The body is interposed between this source and this detector. In practice, it has been shown that moving this point source to a distance of about one meter from the face of the detector could suffice to obtain transmission images. The term "transmission image" herein means that the radiation measured is a radiation that goes through the body from one side to the other, the source of radiation being external: the body transmits (or does not transmit) the gamma radiation. For the reconstruction of tomographic images, in this patent application, it has been shown that it is enough to make a slight correction of the reconstruction algorithms to take account of the conical nature of the radiation.
However, this method has drawbacks. The first drawback is that the patient's body must be subjected to a first irradiation with this external source and that there is therefore a loss of time. The second drawback results from the conical nature of the radiation. Indeed, owing to the limited sizes of the detector, the patient's body cannot be entirely included in the radiation cone. Certain parts of the body, close to the generatrix lines of the cone, are not usefully subjected to the radiation except when these parts are as close as possible to the plane of the detector. For reasons of acquisition of a tomographic image, the gamma camera was rotated by a half-turn about the patient's body. These very same regions are then at the greatest distance from the plane of the detector and are either not irradiated or irradiated, the result of this radiation, however, going beyond the limits of the detector.
Consequently, the reconstruction of the images for these parts which are sometimes taken into account and sometimes not taken into account leads to the creation of artifacts. In practice, it is necessary, for the reconstruction, to limit the operation to the part that is seen whatever may be of the incidence of the gamma camera.
Furthermore, in an article by D. J. Macey et al., "Comparison Of Three Boundary Detection Methods For SPECT Using Compton Scattered Photons, " Journal of Nuclear Medicine, Vol. 29, No. 2, February 1988, a method has been devised for the acquisition of images by transmission by the use of the Compton effect. To this end, this article makes a comparison of the images obtained depending on whether the radioactive source is inside or outside the patient's body and, in the latter case, depending on whether the Compton photons have to be measured at 90.degree. or at 180.degree.. The radioactive marker used is technetium whose radiation energy is 140 KeV. In any case, a series of experiments are undertaken in making the body rotate before the detector of the gamma camera.
The conclusions of this article are that the results of the experiments with internal radioactive source are unusable, whereas they are acceptable when the source is external. In any case, what occurs is not a real transmission but a pseudo-transmission: quite simply, the radiation does not directly come from an injected organ. Hereinafter in the present explanation, the term "scatter" is used to distinguish this phenomenon from "emission" (which is related to the injection of the marker).
For the experiment in which the radioactive marker has been injected into the structure being examined, the energy window opened to measure the Compton photons was centered on 110 KeV and had a width of .+-.15%: from 94 to 127 KeV. In practice, given that for technetium at 140 Kev the lower limit of the energy of the Compton photons taken into account is 90 KeV (with backscattering at 180.degree.), and given that, furthermore, for reasons of detection, it is accepted that a window of .+-.10% around 140 KeV (126 KeV-154 KeV) can be opened to measure the primary gamma radiation, it may be assumed that this experiment has taken account of all the possible Compton photons. Nevertheless, the conclusions drawn by the authors of the article are such that the reconstruction of the contours with an internal source is difficult and even that it is not efficient enough to carry out a quantitative appreciation.
The problem therefore is to be able to do without an external source whose handling is furthermore always dangerous for the handlers, this external source being the cause of loss of time and hence loss of the profitability of the machine.
The idea of the invention however is to use the gamma radiation source as injected into the patient to acquire a scatter image at the same time as the acquisition of the emission image due to the radioactive marker. To this end, in the invention, rather than opening an energy window tending to take account of the primary Compton photons which, in principle, are the most numerous, it has been chosen on the contrary to take account of the secondary Compton photons, or even the tertiary Compton photons, by lowering the energy window so that a substantial part of this window is located beyond the lower limit of emission of the primary Compton photons. For example, for primary Compton photons produced by technetium, this limit is 90 KeV.
In practice, it has been realized that firstly, by acting in this way, a sufficient amount of information is obtained to enable the production of the images and that, secondly, the images produced are not affected by artifacts. It is likely that, with the choice of the invention, the different parts of the body are taken into account more efficiently. This is because the secondary or tertiary Compton photons have locations of emission, locations at which the secondary or tertiary Compton events occur, that are better distributed within the body.
Various tests have been carried out and it has been observed that, provided that the range taken into account includes a substantial part located outside the range of primary radiation and the range of the primary Compton photons, it is possible to obtain a good image.